Method and means for mooring

ABSTRACT

A method and means for mooring and particularly adapted for mooring semisubmersible platforms, such as drilling platforms used in the offshore drilling of oil wells. The invention contemplates providing the platform with a series of tensioned mooring lines, sensing means for sensing tension in the various mooring lines, and means for relieving the tensions on selected mooring lines responsive to the tension on a mooring line rising above its initially tensioned condition due to surge motions of the platform resulting from wind and waves.

United States Patent Inventor Frans V. A. Pangalila Matawan TownshipAppl. No. 813,598

Filed Apr. 4, 1969 Patented May 25, 1971 Assignee John J. McMullenAssociates, Inc.

New York, NY.

METHOD AND MEANS FOR MOORING 9 Claims, 3 Drawing Figs.

US. Cl 1 14/144, 1 l4/0.5 Int. Cl ..B63h25/00, B63b 35/44 Field ofSearch 114/230,

[5 6] References Cited UNITED STATES PATENTS 3,121,954 2/1964 Foster114/144(B)X 3,422,783 1/1969 Moulin Primary Examiner-Trygve M. BlixAttorneyFleit, Gipple & Jacobson ABSTRACT: A method and means formooring and particularly adapted for mooring semisubmersible platforms,such as drilling platforms used in the offshore drilling of oil wells.The invention contemplates providing the platform with a series oftensioned mooring lines, sensing means for sensing tension in thevarious mooring lines, and means for relieving the tensions on selectedmooring lines responsive to the tension on a mooring line rising aboveits initially tensioned condition due to surge motions of the platformresulting from wind and waves.

METHOD AND MEANS FOR MOORING BACKGROUND OF THE INVENTION Thesemisubmersible platform, because of its superior characteristics withregard to motions, is becoming increasingly important to the offshoreoil industry. As commonly used, a semisubmersible platform containingdrilling equipment is positioned over a well site and a riser extendsfrom the platform down to the well site. If the semisubmersible platformwere to move horizontally (surge) more than about percent of the waterdepth, the riser would be subjected to such severe stress that it couldbe in danger of cracking or breaking.

To maintain the semisubmersible platform over the well site and withinthe 5 percent limit of surge movement, a moor is employed consisting ofa series of mooring cables extending from the body of the platform downto the floor of the associated body of water, the cables having apreselected tension called pretension.

It has been found that as the pretension in the cables is increased, sotoo is the external force required to move the platform. Therefore, theplatform may be stabilized by simply increasing the pretension in eachof the mooring cables. However, by increasing the pretension in all ofthe cables, the cables and the platform must be extremely strong.Further, the effects of shock loading (a large increase in tensioncaused by a small movement) are pronounced.

In view of the above, other methods have been devised to ensure that thesemisubmersible platform remains substantially above the drillingsurface. One method which is extensively used in the industry consistsof pretensioning the mooring cables an amount sufficient to stabilizethe platform under normal conditions, and then to manipulate only thewindward cables to counteract the external forces causing surge movementin the platform. However, the additional tension required on thewindward side of the platform is often of such a magnitude that it isnecessary to provide platforms and cables with extremely high strengthcapabilities. For example, if the wind force in -foot waves is 45,000pounds and if the total horizontal component of the forces in theleeward cables is 50,000 pounds, it is necessary that the counteractinghorizontal component of the forces generated by cables on the windwardside be 95,000 pounds. This is the case since the windward cables mustfight" both the external forces and the tension in the leeward cables.

A similar method is disclosed in US. Pat. No. 2,986,889, issued toLudwig on June 6, 1961. This patent discloses a counterweight systemwherein, in response to movement of the platform, the windward cablesare increased in tension while the leeward cables are decreased intension. This system shows some improvement over those systems describedabove. However, the windward cables are still increased in tension andmust therefore be quite strong.

SUMMARY OF THE INVENTION The present invention relates to a method ofmooring semisubmersible platforms and contemplates providing same with aplurality of pretensioned mooring cables spaced about its periphery,means for sensing, comparing and regulating the tension in each of thesemooring cables, and manipulating only the mooring cables whose tensionsare below pretension. More particularly, the tension in the leewardcables is responsively reduced to counteract external forces to maintaintension in respective windward cables substantially at pretension. Theadvantages achieved by the invention are that the cables are operated ina more elastic region, shock loadings are avoided, lower pretensions arepossible, and a smaller chain size can be used to obtain the requiredresults. In practicing the invention, below-pretension cables aremanipulated to ensure that none of the cables substantially exceedpretension. When below-tension cables reach pretension, manipulationstops or shifts to other cables on the platform, and their tension isreduced a suitable amount to maintain new windward.

cables substantially at pretension. In this manner, the platform remainswell within its 5 percent surge limit and is there maintained withoutsubjecting any cable to excessive tensions.

It is therefore the main object of the'present invention to provide amethod and means for maintaining a moored floating body near its restposition in such a manner that none of its mooring lines are subjectedto tensions substantially above the initial tension of the lines. Inthis way, the strength capabilities of the mooring lines and theplatform itself may be minimized, thereby effecting a savings in cost.

It is another object of the present invention to provide a method and ameans for stabilizing a floating body in a manner which is readilyautomated.

These and other objects of the invention, as well as many of theattendant advantages thereof, will become more readily apparent whenreference is made to the following description taken in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a top plan view of asemisubmersible platform hav ing mooring cables associated therewith;

FIG. 2 is a side elevation of the semisubmersible platform shown in FIG.1; and

FIG. 3 is a schematic view of the cable control system forming a part ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION With reference first to FIGS. 1and 2, there is shown a semisubmersible floating platform of the typeuseful in offshore oil drilling. The platform is shown generally at 10and comprises, basically, a drilling platform 12, four legs 14, fourpontoon members 16, appropriate structural members 18 and a riser l9.Attached to the body of the platform 10 and extending from their pointof attachment downwardly to the floor of the associated body of waterare a plurality of mooring cables. Fixedly securing the mooring cablesto the floor of the water body are a plurality of anchors 20.

For convenience of description, the wind direction is taken to be thatindicated by the arrow 22. Also, for ease of description, the cables onthe windward side of the platform 10 are indicated by the numeral 24 andthe cables on the leeward side of the platform 10 are indicated by thenumeral 26. The remainder of the cables, those whose horizontalcomponents lie transverse to the direction of the wind, are indicated bythe numeral 28.

With reference now, to FIG. 3, a system for stabilizing a floatingplatform in accordance with the present invention is schematicallyillustrated. One windward cable is shown at 24. The windward cable 24,which is fixedly anchored to the floor of the water body, passes over aroller 30 and onto a spool 32 driven by a motor 34. Similarly, thecorresponding leeward cable 26 is shown to pass over a roller 36 andwind onto a spool 38 driven by a motor 40. Windward cable 24 and leewardcable 26 are associated with tension sensors 42 and 44, respectively,which sensors continuously monitor the tension in said cables. Thetension sensed by each tension sensor 42 and 44 is transmitted to aninformation station 46. Within this information station 46 are means 48for receiving and comparing tension sensed by the tension sensors withthe value of pretension, direction-sensing means 50 for sensing thedirection in which each tensionis changing and control means 52 forissuing a control signal to appropriate motors 34 or 40 in response tothe value of tension and the direction of change of this tension.

In operation, assuming that the platform starts at its rest position andthe wind is blowing in the direction of the arrow 22, the tension incable 24 starts to rise while the tension in cable 26 starts to fall.Tension sensors 42 and 44 transmit this information to the respectivecomparing means 48 and direction-sensing means 50. Each comparing means48 is programmed so as to issue a response only if the tension in itsassociated cable is below pretension. Therefore, only the comparingmeans 48 associated with the line 26 is active. A signal from thecomparing means 48 serves to gate on the respective direction-sensingmeans 50. As noted above, each direction-sensing means 50 receives asignal from a respective tension sensor. This signal, representative ofthe tension in a cable, is steady, increasing or decreasing. Thedirection sensors 50 are set to issue a signal of a first sense,positive for example, if the tension is increasing and of a second andopposite sense, negative for example, if the tension is decreasing.Assuming that the wind is increasing in velocity, the tension in theleeward cable 26 falls. Therefore, gated on" by its associated comparingmeans 48, the leeward direction sensing means 50 issues a negativesignal, this signal being fed to the control means 52. With the windblowing as noted above, the control means 52 dictates that the spool 33reel out the cable 26. If the wind were decreasing in velocity, theleeward direction-sensing means 50 would issue a positive signal to thecontrol means 52; the control means 52, in turn, would dictate that thespool 38 reel in the cable 26. Each direction-sensing means 50 isfurther equipped to issue a disabling signal which maintains the statusquo of the system if the tension in its associated cable remains atpretension, this disabling signal being fed to the control means 52.

By means of the system described above, platform movement is held to aminimum. This system operates well regardless of the particular externalforces acting on the platform. To illustrate the versatility of thesystems operation, there follows a summary of the stabilizing method ingeneral terms, not being limited to the wind direction shown in thefigures.

The pretension in each of the cables is noted. When any cable tends tosurpass its pretension, as is the case with a windward cable, a cable onthe opposite side of the platform, serving as its counterpart, isdecreased in tension. The decrease in tension of the leeward cable isjust that necessary to maintain the windward cable substantially atpretension. This action stabilizes the platform since the tensionreduction in the leeward cable substantially counteracts the force ofthe wind. No cable goes above pretension by a significant amount.

if, after the operation described in the preceding paragraph isconducted, the wind changes direction or subsides, further operationsbecome necessary-each of these operations involving the manipulation ofa cable which is at or below pretension. If the wind changes directionor subsides, the condition of the cables (after the initial operation ispracticed) tends to cause the platform to move toward the source of theinitial gust of wind. When the platform moves, the initially windwardcable decreases in tension and the initially leeward cable increases intension. In response to the above-described force imbalance, the tensionin the initially leeward cable, presently below pretension, is increasedso that the tension in the initially windward cable remainssubstantially at pretension. This stabilizes the platform, and does sowithout requiring a cable to go over pretension by a significant amount.

If, after the wind subsides or changes direction, and in order tostabilize the platform, it appears necessary to increase the tension inthe initially leeward cable above pretension, a further operationisnecessary. The initially leeward cable is increased in tension, asdescribed in the preceding paragraph, until it reaches pretension. Thenthe place of manipulation automatically shifts and the initiallywindward cable is manipulated so as to maintain the initially leewardcable substantially at pretension. Thus, the initially windward cablebecomes the new leeward cable, the initially leeward cable becomes thenew windward cable and the operation is repeated.

It should be noted that while the present invention has been describedwith reference to a wind direction parallel to two sets of mooringcables and perpendicular to the remaining two sets, the system of thepresent invention works equally well with obliquely directed winds.Further, while the invention has been described with each cable having aseparate control mechanism, a given control mechanism may be employed tosimultaneouslypontrol the tensions in two or more cables.

The present invention has been described with reference to a specificmethod and a specific means for practicing this method. it is evident,however, that numerous alterations, modifications, and substitutions maybe made to the abovedescribed embodiments without departing from thespirit and the scope of the invention. Accordingly, it is the intentthat the present invention not be limited to the above but be limitedonly as defined in the appended claims.

I claim:

1. A method for stabilizing about a preselected position a floatingplatform of the type having anchored mooring lines and comprising thesteps of: pretensioning each mooring line so that the horizontalcomponents of the pretension add to zero when the platform is at saidpreselected position; sensing the tension in each mooring line; andmanipulating only those mooring lines whose tensions are belowpretension in response to the tension sensed so that platform motion isminimized and in such a manner that no line is subjected to a tensionsubstantially above its pretension.

2. The method of claim 1 wherein said select lines are below pretensionand are manipulated in such a manner that they are actively increased intension when the external environment causes an increase in tension, andthey are actively reduced in tension when the external environmentcauses a decrease in tension.

. 3. The method of claim 1 and further comprising the steps of:comparing the tension sensed in each line with the pretension in thatline; and sensing the direction in which the tension changes.

4. The method of claim ll wherein each mooring line is provided with acounterpart mooring line and wherein only the mooring lines whosetensions are below pretension are manipulated, and further comprisingthe step of: manipulating the below-pretension lines so that theircounterpart mooring lines are maintained substantially at pretension.

5. The method of claim 1 and further comprising the step of:discontinuing the manipulation of the mooring lines when the tension insaid select lines remains at pretension.

6. A control unit useful in stabilizing a floating platform of the typehaving anchored and pretensioned mooring lines and comprising: firstmeans for controlling the tension is one set of mooring lines; secondmeans for controlling the tension in a further set of mooring lines,said further set of mooring lines being a counterpart of said first setof mooring lines; and means for sensing and controlling the tension inonly those mooring lines whose tensions are below pretension so thatplatform movement is minimized and so that no line substantiallyincreases above its pretension.

7. The invention as recited in claim 6 and further comprising: means forsensing the tension in each line; means for comparing the tension sensedwith the pretension of each line; means for detecting the direction inwhich the tension of each line changes; and means for controlling teetension in each line in response to the level of the tension and to thedirection of tension change.

8. The invention as recited in claim 6 and further comprising: means forregulating the tension in predetermined mooring lines so as to maintaincounterpart mooring lines substantially at pretension.

9. The invention as described in claim 6 and further comprising: meansfor actively decreasing the tension in a mooring line when environmentalforces cause said line to decrease in tension; and means for activelyincreasing the tension in a mooring line when environmental forces causesaid line to increase in tension.

1. A method for stabilizing about a preselected position a floatingplatform of the type having anchored mooring lines and comprising thesteps of: pretensioning each mooring line so that the horizontalcomponents of the pretension add to zero when the platform is at saidpreselected position; sensing the tension in each mooring line; andmanipulating only those mooring lines whose tensions are belowpretension in response to the tension sensed so that platform motion isminimized and in such a manner that no line is subjected to a tensionsubstantially above its pretension.
 2. The method of claim 1 whereinsaid select lines are below pretension and are manipulated in such amanner that they are actively increased in tension when the externalenvironment causes an increase in tension, and they are actively reducedin tension when the external environment causes a decrease in tension.3. The method of claim 1 and further comprising the steps of: comparingthe tension sensed in each line with the pretension in that line; andsensing the direction in which the tension changes.
 4. The method ofclaim 1 wherein each mooring line is provided with a counterpart mooringline and wherein only the mooring lines whose tensions are belowpretension are manipulated, and further comprising the step of:manipulating the below-pretension lines so that their counterpartmooring lines are maintained substantially at pretension.
 5. The methodof claim 1 and further comprising the step of: discontinuing themanipulation of the mooring lines when the tension in said select linesremains at pretension.
 6. A control unit useful in stabilizing afloating platform of the type having anchored and pretensioned mooringlines and comprising: first means for controlling the tension is one setof mooring lines; second means for controlling the tension in a furtherset of mooring lines, said further set of mooring lines being acounterpart of said first set of mooring lines; and means for sensingand controlling tHe tension in only those mooring lines whose tensionsare below pretension so that platform movement is minimized and so thatno line substantially increases above its pretension.
 7. The inventionas recited in claim 6 and further comprising: means for sensing thetension in each line; means for comparing the tension sensed with thepretension of each line; means for detecting the direction in which thetension of each line changes; and means for controlling tee tension ineach line in response to the level of the tension and to the directionof tension change.
 8. The invention as recited in claim 6 and furthercomprising: means for regulating the tension in predetermined mooringlines so as to maintain counterpart mooring lines substantially atpretension.
 9. The invention as described in claim 6 and furthercomprising: means for actively decreasing the tension in a mooring linewhen environmental forces cause said line to decrease in tension; andmeans for actively increasing the tension in a mooring line whenenvironmental forces cause said line to increase in tension.